The feedback on this page should serve as a warning to any developer who wants to start charging for an app they previously offered for free. Many people are ungrateful and think, though they get paid for their work, you have no right to do the same. One may quibble over the price, but the principle remains. I've seen complaints when an app went from free to just $5. You may think you can do without such people as customers, but they will frag you on websites like this one anyway. They seem to miss the irony that they are being every bit as mean spirited as they accuse you of being. It's a clear case of transference. They think they'll look better by ragging on you. They're wrong.
If an app worked well enough to get a four star rating when it was free, it's utility doesn't drop significantly if it now costs a little or a lot. Cheap and expensive are subjective values at best, having nothing to do with how well an app works. Thus, without even trying it, I'll give it four stars based on the majority of reviewers who like it.